In hospitals hypodermic syringes and small vials are frequently partially filled with a medicament. The concentration of the medicament is then altered by injecting a measured volume of a diluent, such as sterile water or normal saline. These diluents, and sometimes the medicaments themselves, are supplied to the hospital pharmacy in large bulk containers.
In the past, the pharmacists had used a syringe pumping system with two check valves to pump a measured dose of diluent or medicament to a series of hypodermic syringes or vials. Such syringes or vials could then be transferred from the pharmacy to the hospital floors for injection into patients.
A problem with these syringe pumping systems involved the check valve system. Because the reservoir was hung mouth downwardly from a height of 2-3 feet above the check valve to insure proper drainage, a pressure head in this amount was created at the check valves. To prevent leakage at the check valves, one system proposed using very strong coil springs to provide a very strong closing force at both the inlet and outlet check valves. If a spring were provided to assist on a rearward filling stroke of a metering syringe to overcome the high bias of the inlet check valve, a forward stroke of the metering syringe had to overcome this large spring force as well as the heavily biased outlet valve. Numerous manual strokes of the metering syringe to overcome this large resistance could cause operator fatigue.
To overcome the operator fatigue problem it has been suggested to use weakly biased duckbill valves as the inlet and outlet check valves. To overcome a leakage problem through such weakly biased duckbill valves, it was proposed to place a small piece of foam material wedged across the openable slot of the inlet duckbill valve. A duckbill valve has two rubber members that open and close against itself somewhat like the action of a duck's bill. A duckbill valve does not require engagement and disengagement from a seat on a rigid valve body.
It would appear that the small piece of foam material wedged against the openable slot of the duckbill valve could readily get wedged into such slot holding it continuously open. To our knowledge, the double duckbill valve system has never been marketed with the small piece of foam material across the inlet duckbill valve.